Calpactin I, one of the EDTA-extractable proteins of the lens membrane, binds phospholipid and actin in a calcium-dependent manner. It is also known substrate of the pp60arc kinase. Analysis of embryonic chicken lens RNA with a bovine calpactin I-specific cDNA probe revealed the presence of a approximately 1.8 Kb calpactin mRNA in the lens cells. Six-day embryonic chicken lenses were microdissected into central epithelium, equatorial epithelium, and fiber cells. Total cytoplasmic RNA was isolated from these samples and calpactin I mRNA levels were determined by the polymerase chain reaction (PCR) following reverse transcription (RT). Quantitative PCR indicates that the calpactin I mRNA levels in the equatorial epithelium are greater than in the central epithelium by a factor of 12.7 +/- 2.7. Calpactin I mRNA in fiber cells is an additional 3.5 +/- 1.5 times greater than in the equatorial epithelium. Whole mounts of embryonic chicken lens epithelia and histological sections of whole lenses were also examined with an antibody directed against chicken calpactin I. Calpactin I was predominantly localized in a punctate distribution in equatorial epithelial cells and near the plasma membrane of elongate fiber cells. The elevated levels of calpactin I mRNA observed in the equatorial epithelium and fiber cells and the immunological localization of the protein suggest a possible role of calpactin I in the elongation of fiber cells during lens differentiation.